Fire test manikin
Published: 01 June, 2007
Following an ignition in a fireworks production facility, HSL (Health and Safety Laboratory) undertook work to quantify the size, duration and thermal emissive power of fireballs produced when different quantities of a range of pyrotechnic compositions are ignited.
This study was followed by an equivalent investigation relating to propellants.
Work of this nature can be used to estimate the thermal threat to exposed workers and also to produce possible estimates of burn injuries. However, as a result of risk assessments, steps are generally taken to reduce this type of exposure. Engineering controls (such as remote working, barriers and screens) are the preferred method since they offer the best protection. In those circumstances where engineering controls are not reasonably practicable, PPE is used as a last resort.
Since the commercial UK explosives industry is not particularly large, specialised PPE is generally unavailable and fire protective clothing for other hazardous industries (e.g. oil, offshore) is therefore often utilised.
Little work has been done to assess the effectiveness of this PPE against the heat outputs from burning explosives (which generally produce an intense, short duration event).
This situation was reviewed some years ago by test house BTTG with assistance from HSL and the need was identified to develop appropriate test methods to complement the thermal output studies. As an interim measure, the Confederation of British Industry (CBI), with contributions from HSE (Health and Safety Executive), HSL and BTTG, published, via its Explosives Industry Group, a guide entitled ‘Fire Protective Clothing’. This document recognised that work was needed to test and rank protective clothing and related PPE when exposed to burning explosives. As a result, HSE commissioned HSL and BTTG to jointly develop and use an instrumented manikin to evaluate PPE for the explosives industry.
The manikin test system
The design concept for this instrumented manikin involved assessing not only the effects of frontal exposure of a worker at a simulated workbench containing burning explosive material but also the effects of exposure to the back of a worker escaping from the workbench.
Realisation of this concept was achieved by installing the heat-sensing manikin on a turntable attached to a trolley running on rails creating a 5.5m length of ‘escape’ track. The motion of the manikin is controlled by electric motors programmed as to when to start to retract the manikin after ignition of the explosive material on the workbench, when to turn the manikin through 180° and when, and at what rate, to accelerate it away from the workbench and along the track.
It is therefore possible to programme the manikin to react realistically to a sudden ignition of an explosive material typical of those encountered in the explosives industry.
A commercial source of one manikin around which the EN ISO specification is written was identified in the USA. Manikins from this source are well-established as the basis for ‘Thermo-Man’ at the DuPont Company sites in the USA and Switzerland. The manikin obtained has the same number and location of heat sensors as other manikins from this source (120), is made from the same flame-retardant epoxy resin/glass fibre material and has the same dimensions and stance.
The heat sensors (essentially thermocouples) are located over 88% of the body area including the head, legs and arms but excluding the hands and feet. They are mounted very slightly proud of the manikin surface so that in regions of the manikin where the test clothing and any other PPE is in contact with the manikin surface, it is also in contact with the sensors. The ‘hands’ of the manikin are currently without fingers or sensors as this is a technical challenge beyond the scope of this particular project but is an aspect that could be addressed in the future.
The principle of the proposed EN ISO standard applicable to this moving manikin is to continually record the sensor responses during each test using a data logger (installed in the chest cavity of the manikin). Post-processing of data from the sensors involves utilising the approach proposed in the EN ISO standard to predict skin burns which can be categorised as:
; First degree burn – burn damage to the epidermis layer
; Second degree burn - partial thickness burn to the dermis layer
; Third degree burn – full thickness burn to the dermis and onset of damage to the underlying subcutaneous layer
The results are therefore expressed as the percentage of the area of the manikin that is fitted with sensors (88%) receiving heat which is predicted, by using equations in the EN ISO standard, to indicate 1st, 2nd or 3rd degree burns. These predictions are presented as a body map of the front and back of the manikin divided into colour-coded regions to identify the locations/levels of burn injury prediction or regions of no prediction. It is generally accepted that results are expressed as a cumulative percentage of the body predicted to receive combined 2nd and 3rd degree burns, these being the burns that are medically significant.
The finalised test system was designed specifically to be portable, enabling it to be set up at sites other than HSL, such as an explosives industry site (outdoors or in a suitable building), a fire brigade training facility (in front of a fire wall rig or “flashover” fire container rig) or another suitable location in which it can achieve exposure to a real event. The system therefore helps to bridge the gap between the laboratory-based procedure of the EN ISO manikin standard (featuring a gas burner array simulating ‘flashover’ fire energies as might be experienced by firefighters) and real events that can cause fatalities. With respect to its use by or for the explosives industry, portability is essential because of restrictions on the transport of explosive materials and their manufacture only at suitably licensed sites.
The test programme
The manikin motion programme selected for all of the tests was based on documented human reaction times to fireball radiation and the typical speed at which people escape. The manikin was therefore initially held stationary for 2.0 seconds (s) at a waist-high 0.9 metres (m) workbench after ignition of an explosive material located on the bench. It was then retracted for 1.0 s while still facing the workbench and then continued to retract while making a 180° turn over the next 1.0 s. The manikin subsequently continued along the 5.5m length of rail at a velocity of 2.5m/s, braking quickly to a stop over the last 0.5m.
Details of the explosives used to provide the thermal challenges to the PPE in this test programme are listed in Table 1. The principal items of clothing and headwear are listed in Tables 2 and 3.
Triplicate tests were generally performed in order to provide mean data and to minimise the effects of localised wind velocity on the shape/movement of the fireballs resulting from ignition of these three types of explosives materials.
The final series of tests featured Magnesium Teflon Viton explosive material (MTV) as used for infra-red decoy flares released by aircraft defence systems to divert heat seeking missiles. These tests were undertaken in a large enclosed test facility at the Wallop Defence Systems site.
Summary of test results – clothing
The results (Table 4) show that, when the four types of coveralls were tested against the Firearm propellant, Flare and Star compositions, the extra protective effect of using a two-layer coverall (to effectively double the weight and thickness of the garment) is appreciable and consistent, producing the best results for each test composition.
These results also show that the protection provided is significantly affected by the type of explosive composition, even when these were used in equal quantities and form (as loose powder). Burn injury predictions from Firearm propellant show that each of the four types of coverall is almost certain to prevent serious injury whereas with Star composition all four types of clothing displayed unacceptable, life-threatening results.
The effect of the fibre composition of the three types of single layer coverall is less clear but the results suggest that, overall, the aramid (Nomex lll) coverall is likely to provide slightly greater protection than the two FR treated coveralls. However, more testing, taking into account coverall design and fit, would need to be undertaken to reach a more certain conclusion.
The final series of clothing tests were intended as a start to studies of the protection currently provided against the very high temperature and rapid burning of MTV infrared decoy flare material, accidental ignitions of which have resulted in fatalities. The results, again expressed as mean values from triplicate tests of the clothing systems, clearly suggest that the use of leather outer garments can provide greater protection than aluminised outer garments. All results were less than 10% 2nd and 3rd degree burn injury predictions.
The purpose of this initial comparison was to determine if the radiant heat reflective performance of aluminised outer garments which, according to laboratory test data, is very high compared to leather or textile garments, would be carried through to this comparison of in-use, complex multi-layer, multi-garment PPE ensembles. The initial indications are that this is not the case but the reasons as to why will require more study. It is, however, probably linked in part to the heat energy emitted by MTV, i.e. the proportion of radiant to convective heat actually received by the PPE ensemble.
Summary of results – headwear
These results, again expressed as mean value burn injury predictions from triplicate tests are set out in Table 5. Please note that the small number of heat sensors in the head region of the manikin causes the results to have to be expressed in steps of approximately 17%, e.g. 0%, 17%, etc. With this proviso, the results do suggest that, the Wool/FR Viscose firehood provided the best protection out of the four types of firehood tested. Similarly to the clothing tests, these results will be influenced by other factors than simply the type of fibres and their proportions used in these four firehoods. The important conclusion from these initial tests is that all the results are indicative of very severe burn injuries resulting from ignition of 5kg quantities of the three explosive materials. This outcome contrasts unfavourably with the results for coveralls when worn with these firehoods, particularly in the case of Firearm propellant. However, the results do again show the same trend with respect to the type of explosive material, the results with Star composition always being worse than those with Firearm propellant.
The two sets of tests with MTV indicated, not surprisingly, that the ensemble incorporating an airstream helmet provided slightly superior protection, good enough for zero 2nd and 3rd degree % burn injury predictions with the proviso set out above regarding minimum % burn injury prediction intervals.
Conclusions
Initial experience with this innovative moving manikin fire test facility demonstrates that it can be used to produce test data from realistic explosive material ignition events that is logical, repeatable and therefore of real value to users and manufacturers of PPE that is intended to provide protection against specific fire threats.
The results reported in this article tend to support the trends seen in test data from static, gas flame engulfment manikins such as BTTG’s long-established RALPH (Research Aim Longer Protection against Heat) and recently introduced SOPHIE (System Objective Protection against Heat In Emergencies). For example, the results from exposure to three types of realistic event (ignition of different explosive materials in quantities handled by operatives in the explosives industry) show clearly the benefit of increasing the weight and thickness of protective coveralls and also suggest that for protection against such high energy/high temperature events, garments made from typical aramid fibres are slightly superior to those made from typical FR treated fabrics.
The test equipment is kept at HSL, Buxton, the owners, and is operated by HSL in conjunction with BTTG. It is available commercially to undertake test programmes for clients either at HSL or, if appropriate, a customer’s site.
The tests were made possible by the assistance of the following: Wallop Defence Systems Ltd., Middle Wallop, Wiltshire (the site for some of the test programme); Chemring Countermeasures, High Post, Wiltshire; the Institute of Naval Medicine, Alverstoke, Hampshire; Black Cat Fireworks Ltd. (formerly Standard Fireworks Ltd.), Huddersfield, West Yorkshire.
